Electrostatically focused transverse field backward wave amplifier



Nov. 12, 1968 P. G. EVERETT ELECTROSTATICALLY FOCUSED TRANSVERSE FIELD BACKWARD WAVE AMPLIFIER Filed Feb. 21, 1966 Ila III '61 g Electron Input Line Input Line- Gun .T m w R y we m m fi v m N 1 EA r 9 I 9 D1 Ffa. 3

United States Patent 3,411,100 ELECTROSTATICALLY FOCUSED TRANSVERSE FIELD BACKWARD WAVE AMPLIFIER Peter G. Everett, Oak Park, IlL, assignor to Zenith Radio Corporation, Chicago, Ill., a corporation of Delaware Filed Feb. 21, 1966, Ser. No. 528,964 14 Claims. (Cl. 33043) The present invention pertains to backward-wave electron devices. More particularly, the invention relates to backward-wave electron couplers and to amplifiers employing such couplers.

In the conventional longitudinal-mode travelling-wave tube, an electron beam is projected close to a circuit which propagates a signal Wave. When the electron velocity has a value somewhat greater than the velocity of the wave on the circuit, coupling or interaction between the beam and the circuit wave occurs with resultant amplification of the signal. Most often, the electrons in the beam are prevented from spreading by means of a magnetic field produced by an enveloping solenoid or permanent magnet structure.

Another known device is the transverse-field travellingwave tube which typically includes a pair of transmission lines or the like disposed on opposite sides of an electron beam. Similarly to the longitudinal-mode tubes, interaction occurs between signal waves travelling on the transmission lines and the beam with resultant gain in signal level. Interaction between the travelling-wave transverse signal field and the stream of electrons has been analyzed in chapter 13 of the book entitled Travelling-Wave Tubes, J. R. Pierce, published by D. Van Nostrand Co., Inc., New York, 1950.

A particularly attractive form of the transverse-field travelling-wave tube is one in which the electron beam is focused by a series of electrostatic electron lenses, dispensing entirely with the need for a magnetic focusing field. Exemplary of such tubes are those disclosed in US. Letters Patents Nos. 2,809,320, 2,866,916 and 2,878,413, all issued to R. Adler and assigned to the same assignee as the present application. Because the electric signal field is transverse to the direction of electron beam flow, noise reduction techniques, inapplicable to the more conventional longitudinal-mode devices, may be utilized. Additionally, substantially higher electron beam currents are made possible in that the electron beam may be in the form of a sheet or ribbon instead of being cylindrical like a pencil.

In the aforesaid now generally conventional travellingwave tubes, the signal energy on the transmission line or circuit usually travels in the same direction as the stream of electrons, that is, from the electron gun to the ultimate electron collector. Some attention, however, has been given to the backward wave amplifier in which the circuit power travels in a direction opposite to that of the electrons. While the wave on the transmission line thus is a backward wave, the electric field peaks as seen by the electrons travel in the same direction as do the electrons. As in the more conventional forward-wave travellingwave tube, there is a specific electron velocity for which signal gain occurs. Also as is the case with the forwardwave devices, the essential primary element is a circuit disposed alongside the electron stream and which propagates a signal wave with a velocity appropriately related to that of the electrons in the beam.

It is a general object of the present invention to provide a new and improved device for producing interaction between a signal wave and an electron beam.

Another object of the present invention is to provide an interaction device particularly suited for backwardwave operation.

A related object of the present invention is to provide a new and improved voltage-tunable wave amplifier.

Electron interaction apparatus according to the present invention includes means for projecting an electron stream along a path together with field-producing means for establishing a condition of transverse resonance for the electrons in the beam. Spaced successively along the electron beam path are a plurality of pairs of electrodes with the individual electrodes of each pair being disposed respectively on opposite sides of th path and with each electrode being capacitively coupled to the other of its pair and to a corresponding one of the electrodes in each adjacent pair. Inductor means individually inductively couples together the individual electrodes of each of the pairs. Coupled across the individual electrodes of an end one of the electrode pairs is signal translating means with the individual electrodes and the aforesaid capacitive and inductive coupling together constituting a transmission line propagative of the signal with a wave velocity so related to th average velocity of electrons in the stream as to effect interaction between the stream and fields created by the signal on the transmission line.

The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The organization and manner of operation of the invention together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals refer to like elements in the several figures and in which:

FIGURE 1 is a schematic diagram of an amplifier embodying the present invention;

FIGURE 2 is a fragmentary perspective view, partially schematic, of an electron interaction device utilized in FIGURE 1; and

FIGURE 3 is a schematic diagram of an equivalent circuit at signal frequencies of the device shown in FIG- URE 2.

FIGURE 1 illustrates a complete backward wave amplifier having an electron gun 10 from which an electron beam or stream is projected along a path 11 terminating in a collector 11a connected to a suitable positive potential source B+. Disposed alongside a portion of path 11 first downstream from electron gun 10 is a transmission line structure composed of two portions 12 and 13 individually disposed on opposite sides of path 11. Signal translating means in the form of leads 14 and 15 serve to couple a signal source 16 across the downstream end of portions 12 and 13 of the transmission line A load is matched and coupled to the upstream end of this transmission line and as illustrated takes the form of a matching pair of terminating resistors 17 and 18 individually coupled between the ground plane of reference potential and the respective upstream ends of transmission line portions 12 and 13.

Further downstream beyond transmission line 12, 13 is a second transmission line composed of portions 20, 21 individually disposed on respective opposite sides of beam path 11 and, as embodied herein identical with transmission line portions 12, 13. At their downstream ends, portions 20 and 21 are terminated in a match load composed of respective resistors 22 and 23 coupled to those downstream ends in the same manner as is the case with resistors 17 and 18. A signal load 24 is coupled across the upstream ends of transmission line portions 20 and 21 by leads 25 and 26 which translate the signal from the line to the load. Isolating the fields produced by input transmission line 12, 13 from those produced by output transmission line 20, 21 is a shield electrode 27 disposed between the two transmission lines and having an aperture centered on beam path 11.

In overall operation, the signals from source 16 are launched backwardly along input transmission line 12, 13

and ultimately are absorbed in the terminating load 17, 18. The backwardly travelling signal wave creates an electric field having peaks which travel in the direction of electron flow. Upon proper adjustment of the average electron velocity, gain of the signal-wave energy occurs. The electron stream leaving transmission line 12, 13 carries amplified signal energy as an electron wave on the beam.

In the output portion of the device further downstream that signal energy on the beam interacts with transmission line 20, 21 to cause signal waves to be developed on the latter. As in the input section, the velocity of the circuit wave travelling on transmission line portions 20, 21 is so related to the average electron velocity as to cause interaction between the signal fields on the line and the beam with resultant further amplification of the wave travelling along the transmission line. This amplified wave energy ultimately is derived from the upstream end of transmission line 20, 21 by load 24. Transmission line 20, 21 again is a backward wave device wherein the electron field peaks as seen by the electrons travel in the direction of electron flow.

Since in this embodiment the input and output transmission lines are identical in construction, it will suffice for purposes of detailed description to direct further at tention to but one of them. Accordingly, FIGURE 2 depicts the input transmission line having portions 12 and 13. The transmission line takes the form of a plurality of pairs of electrodes 30-38 and 40-48 spaced successively along path 11 with the individual electrodes, e.g. 30, 40, of each pair being disposed respectively on opposite sides of path 11. By virtue of the physical spacing between the different electrodes, each electrode is capacitively coupled to the other of its pair, as electrode 30 is capacitively coupled to electrode 40, and each electrode also is capacitively coupled to a corresponding one of each adjacent pair as electrode 31 is capacitively coupled to each of electrodes 30 and 32. Coupling portions 12 and 13 together are a plurality of inductors 50-58 individually connecting together the electrodes of each successive pair of electrodes; for example, inductor 50 is connected between electrodes 30 and 40. Leads 14 and 15 are connected respectively to electrodes 48 and 38 of the pair furthest downstream. Similarly, leads 60 and 61 serve to couple electrodes 30 and 40 of the pair furthest upstream respectively to terminating resistors 17 and 18. As illustrated, electrodes 30-38 and 4048 are flat plates of conductive material afiixed in position upon longitudinally extending insulating rods 63 extending through the electrodes and conventionally secured at opposing ends of the usual enclosing glass envelope (not shown).

The equivalent signal-frequency circuit of FIGURE 2 is shown in FIGURE 3. It will be seen that the transmission line constitutes a plurality of band pass filter sections each composed of a shunt inductor in parallel with a shunt capacitor with the sections being capacitively coupled one to the next. The values of capacitance are functions primarily only of the facing width of the individual plates and of their spacing from each adjacent plate.

In order to confine the electron beam throughout its travel and to condition it to accept signal energy from input line 12, 13 and translate the signal to output line 20, 21, the apparatus further includes field producing means for establishing a condition of transverse resonance for the electrons in the stream, To this end, a succession of electrostatic lenses is created by coupling alternative different static potentials individually to respective successive pairs of the electrodes. Thus, a first source of potential B is coupled to the even electrode pairs 30-40, 32-42 and so on while a second potential source B is coupled to the odd electrode pairs 31-41, 3343 and so on. Conveniently, source B is connected to an intermediate center tap on the even numbered ones of inductors 5058 while source B is connected similarily to the center taps on the odd numbered ones of those inductors.

The difference of potential as between sources B and B together with the spacing between the successive pairs of electrodes create the succession of electrostatic lenses which in turn establish the condition of transverse electron -resonance. Each of the periodic lens fields tends to deflect the electrons toward reference path 11 with a force which is proportional to the displacement of the electrons from that path. Accordingly, the lens field is generally equivalent to a transverse elastic field and tends to confine the electrons to a maximum stream width.

The focal length of each of the electrostatic lenses formed between adjacent pairs of the electrodes is proportional to the transverse spacing between those electrodes and is a function of the ratio between the electrostatic potentials on the electrodes with respect to the reference potential of electron gun 10. The focal lengths of the electrostatic lenses, in conjunction with the spacing between adjacent lenses, determines the distance along path 11 which is traversed by an electron having a given initial transverse velocity before that electron crosses path 11. Consequently, each electron follows a substantially sinusoidal path which is symmetrical with respect to reference path 11. These sinusoidal trajectories have a wavelength A Moreover, the magnitude and direction of any initial transverse velocity and the original displacement of the electron with respect to path 11 does not affect that wavelength.

For a given structure, the resonance wavelength 1,, is determined by the strength of the lens field produced by the electrostatic potential difference between the successive electrode pairs and by the average of the individual potentials on those pairs which establishes the average velocity of the electron stream. As each electron follows its individual trajectory, it carries out a transverse harmonic motion at a frequency w equal to its average velocity divided by the resonance wavelength 1,. Consequently, the electron stream is said to exhibit a transverse resonance at the frequency w By proper choice of the unidirectional potentials applied to the transmission line structure, the velocity of the electron stream may be adjusted so that:

e o( e o) (1) where to is the frequency of the signal applied from source 16, v is the propagation velocity of the undisturbed signal wave travelling along the transmission line and v is the average velocity of the electrons. Under these conditions, the individual electrons in the stream are sub jected to a periodically changing signal field having a frequency w that is, the field produced by the signal of frequency w appears to the electrons to have a frequency w Since the electrons tend to resonate at the frequency w they begin to move at increasing amplitudes in the direction transverse to beam path 11. The electrostatic lens fields created by sources B and B are sufficiently strong in comparison to the signal wave field that the transverse excursions of the electrons induced by the signal field do not cause the electrons to be collected by the transmission-line electrodes.

It is further contemplated to adjust sources B and B together in order to vary the average static potential on the electrodes and consequently change the center frequency of response to the signal. It has been found that, for the backward waves utilized, the wave velocity on the transmission line circuit changes rapidly with frequency, that the instantaneous bandwidth is small and that the center signal frequency is a function of the axial electron velocity which in turn is proportional to the unidirectional voltage of the electron beam. Consequently, each section of the amplifier of FIGURE 1 and the amplifier itself exhibit the characteristic of a voltage tunable filter, a filter which imparts gain to the signal. It has been found 5 experimentally that the center frequency f varies with beam voltage V in accordance with the relationship:

where V is the voltage at the lowest frequency f,,.

An exemplary backward wave amplifier constructed as described has an approximate length of 6 inches and an envelope diameter of 1.25 inches. Its weight is 5 ounces. In use in the form of an octave tunable bandwidth amplifier, leads 14 and 15 constitute a 300 ohm balanced line; when in the form of a 20 percent bandwidth amplifier, leads 14, 15 are either 300 ohm line or, utilizing a balun converter, 50 ohm coaxial line. In both versions, the V.S.W.R. is better than 1.2 over the entire pass band. The gain at center frequency is between 20 and 30 db.

The lower limit of dynamic range is defined by the noise figure, while the upper limit is defined by current interception with the circuit electrodes at large signal amplitude. Saturation occurs gradually as in the case of the conventional forward-wave travelling-wave tube and the power level exhibited is similar to that in cyclotron-wave electron-beam parametric amplifiers.

It has been found that isolation between the output and input sections can be essentially as high as necessary. In addition to shield electrode 27, it may be desirable in some environments to add a mu metal magnetic shield of 0.040 inch thickness over the envelope. Isolation of greater than 60 db has been exhibited. Noise figures of the order of 4 db are readily obtainable and, with due attention to beam formation in the electron gun, it appears that noise figures below 3 db are to be expected.

The basic bandwidth is about 1 percent of center frequency. It has been found that the instantaneous frequency response is symmetrical about the center frequency f and that the product of gain and bandwidth is essentially constant. It appears that the incremental instantaneous frequency response varies by a factor of about 2 over the tunable frequency range, being greater at the higher frequency end; the latter implies that the percentage inst-ant-aneous bandwidth tends to be substantially constant over the tunable band. At any given frequency, the maximum instantaneous bandwidth is limited to a maximum of about 5 megacycles. The minimum bandwidth can be as small as desired, but at any one frequency the tube length doubles each time the bandwidth is halved.

With respect to the voltage-responsive tuning characteristic of the device, it has been found that for the embodiment wherein the bandwidth is 20 percent of the center frequency, the voltage swing for tuning over the full frequency range is about 1.5 to 1. For the embodiment having a octave bandwidth, that voltage swing is about to 1. The actual tuning voltages required for these embodiments operating in the UHF region are approximately from 100 volts to 150 volts for the 20 percent band width amplifier and from 50 volts to 500 volts for the octave bandwidth version.

Further, it can be shown that for any given obsolute value of frequency change, the fractional change of voltage is approximately a constant. For example, in a tube the tuning of which is adjustable from 500 to 1,000 megacycles, a 5 megacycle change in center frequency requires a 2.3 percent change in voltage anywhere within that band. The same percentage change in voltage would also be required for adjusting the frequency by that amount in a tube having a tuning range of only 500 to 600 megacycles. Of course, the speed by which the frequency may be swept through the range depends in large part upon the capacity presented to the DC potential circuit. Another characteristic of interest is the variation of gain as the frequency is adjusted by varying the tuning potential. Such gain variation approximates less than three db for the octave-range amplifier and of the order of one db for the 20 percent bandwidth amplifier.

As has been shown, the signal frequency circuit as depicted in FIGURE 3 propagates a backward signal wave which creates .a uniform electric field in the region of the beam which field interacts with the fonward-moving electrons to produce gain. At the same time, the electrostatic fields established by the series of electron lenses illustrated in FIGURE 2 create the requisite periodic electrostatic focusing. The properties of the band pass filter circuit which is formed in accordance With FIGURE 3 are such that it has the phase shift characteristic of a backward wave structure. This means that the phase of the rotating alternating current signal vector advances with each section in the direction of the power flow. Consequently, the resultant phase velocity is in the direction opposite to the power flow, much as the phase velocity of the thread Wave of a screw being turned into a nut is in a direction opposite that of the direction of power flow causing the screw to become further engaged with the nut.

In the complete two-section amplifier of FIGURE 1, an exemplary embodiment utilizes an electron gun which develops an electron beam about one-half inch wide by 0.010 inch thick. In one sense, the input and output sections operate like the input and output cavities of a klystron. Signal power fed into the downstream end of the initial backward Wave circuit interacts with the beam and bunches it. The circuit wave, amplified in the exemplary embodiment by perhaps 10 db, is then absorbed in the matched load at the upstream end of that section. Further downstream, the consequently bunched electrons enable interaction between the beam and the second section. This induces signal power into the latter which is derived at its upstream end. The resultant overall gain is approximately 20 db over the original input signal.

The purpose of the matched termination loads on both of the sections is to eliminate self oscillation which otherwise may occur at high gain levels by reason of imperfect matches at the signal input and output terminals. By virtue of the use of the terminating loads and the incorporation of shield 27, the resultant isolation between the output and input sections is very good, being greater than 60 db, and this is achieved at a cost of only negligible overall loss in gain.

As in any conventional amplifier, the embodiment of FIGURE 1 exhibits a signal to noise ratio at the output which is lower than the corresponding ratio at the input. This occurs because the random positions .and velocities of the electrons in the beam behave like an input signal and interact with the circuits to produce noise at the output. In practice, the magnitude of this beam noise is reduced by keeping the beam narrow and by slicing out the high-velocity ends of the Maxwellian electron velocity distribution. Both of these techniques are incorporated by utilizing effective gun design, following teachings by now well understood in the .art. However, even using an electron gun of a much earlier design than is available at the present, noise figures of less than 5 db have been measured in an amplifier of the FIGURE 1 type.

One approach for further reducing noise is to include a converging electron lens located near the cathode in electron gun 10. Electrons with the same velocity will cross path 11 at the same point. Further incorporating a narrow slit placed at the cross over point to intercept electrons that have other than the desired velocity, the velocity fluctuations and consequently the noise temperature of the beam are reduced. For example, utilizing a dispenser cathode having a temperature of 2,000 degrees K. with a beam thickness of 5 mils .at the cathode together with an aperture or slit that intercepts percent of the beam, the noise temperature of the beam beyond the cathode is less than 300 degrees K. over a frequency range from 450 to 900 megacycles.

From the foregoing, it is evident that devices constructed in accordance with the invention are suitablefor a Wide variety of applications. The simple voltage-responsive frequency selection is particularly suited for uses such as in UHF television tuners where a full video bandwidth must be accommodated and where the range of channel frequencies is comparatively large. At the same time, such amplifiers are completely self contained, requiring no external focusing solenoids or the like. It has .also been shown that the amplifiers are quite suitable for a number of other applications requiring much greater percentage bandwidths.

While particular embodiments of the present invention have been shown and described, it is apparent that changes and modifications may be made without departing from the invention in its broader aspects. The aim of the ap pended claims, therefore, is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. Electron interaction apparatus comprising:

means for projecting an electron stream along a path;

field-producing means for establishing a condition of transverse resonance for the electrons in said stream; a plurality of pairs of electrodes spaced successively along said path with the individual electrodes of each pair being disposed respectively on opposite sides of said path and with each electrode being capacitively coupled to the other of its pair and to a corresponding one of the electrodes in each adjacent pair;

inductor means for individually inductively coupling together the individual electrodes of each of said pairs;

and signal translating means coupled across the individual electrodes of an end one of said pairs, said electrodes and the aforesaid capacitive and inductive coupling together constituting a transmission line propagative of said signal with a wave velocity so related to the average velocity of electrons in said stream as to effect interaction between said stream and fields created by said signal on said line.

2. Apparatus as defined in claim 1 in which said translating means is coupled to the downstream end pair of said electrodes and the signal wave propagates on said line in the upstream direction, said signal Wave creating electric field peaks which, as viewed by said electrons, travel in the downstream direction.

3. Apparatus as defined in claim 2 in which said signal translating means is responsive to an input signal and further comprising:

a second plurality of pairs of electrodes like and disposed downstream of the first;

a second inductor means like the first;

and a second signal translating means like the first and coupled to means for accepting an output signal.

4. Apparatus as defined in claim 3 including means disposed along said path between the first and said second plurality of pairs of electrodes for isolating one from the other.

5. Apparatus as defined in claim 1 including a load matched to said line and coupled to the individual electrodes of the other end one of said pairs.

6. Apparatus as defined in claim 1 in which said fieldproducing means includes first and second sources of individually different static potentials coupled individually to respective successive pairs of said electrodes to create a corresponding succession of electrostatic lenses.

7. Apparatus as defined in claim 6 in which said fields created by said signal on said line are uniform in the region of said stream.

8. Apparatus as defined in claim 6 in which said first and second sources are adjustable together to vary the average static potential on said electrodes and change the center frequency of response by said line to said signal.

9. Apparatus as defined in claim 6 in which said inductor means includes individual inductors coupled between the respective individual electrodes of said pairs with said inductors having intermediate taps and with said first and second sources coupled individually to corresponding respective ones of said taps.

10. Apparatus as defined in claim 9 in which the ones of said inductors coupled to said first source are disposed on one side of said path and the others of said inductors coupled to said second source are disposed on the opposite side of said path.

11. Apparatus as defined in claim 1 including adjustable means for impressing a static average potential on said electrodes, a change in said average potential varying the center frequency of response by said line to said signal.

12. Apparatus as defined in claim 1 in which each of said electrodes is a fiat plate disposed in a plane transverse to said path and defined by the other electrode of its pair and parallel with the planes defined by the others of said pairs.

13. Apparatus as defined in claim 1 wherein the electrodes of each pair define a space through which said stream is projected and which is elongated in a direction transverse to said path and wherein said stream has a transverse cross-section which is elongated in said direction.

14. Apparatus as defined in claim 1 in which said transmission line constitutes a band-pass filter, the signal pass band of said line being a function of the ratio of series to shunt capacitance between said electrodes.

References Cited UNITED STATES PATENTS 2,785,338 3/1957 Goddard 3l539 NATHAN KAUFMAN, Primary Examiner. 

1. ELECTRON INTERACTION APPARATUS COMPRISING: MEANS FOR PROJECTING AN ELECTRON STREAM ALONG A PATH; FIELD-PRODUCING MEANS FOR ESTABLISHING A CONDITION OF TRANSVERSE RESONANCE FOR THE ELECTRONS IN SAID STREAM; A PLURALITY OF PAIRS OF ELECTRODES SPACED SUCCESSIVELY ALONG SAID PATH WITH THE INDIVIDUAL ELECTRODES OF EACH PAIR BEING DISPOSED RESPECTIVELY ON OPPOSITE SIDES OF SAID PATH AND WITH EACH ELECTRODE BEING CAPACITIVELY COUPLED TO THE OTHER OF ITS PAIR AND TO A CORRESPONDING ONE OF THE ELECTRODES IN EACH ADJACENT PAIR; INDUCTOR MEANS FOR INDIVIDUALLY INDUCTIVELY COUPLING TOGETHER THE INDIVIDUAL ELECTRODES OF EACH OF SAID PAIRS; 